1. Technical Field
The invention relates generally to MOS based memory cells. More specifically, the invention relates to the improvement of leakage of memory cells using deep submicron MOS transistors.
2. Description of the Prior Art
Conventional complementary metal-oxide semiconductor (CMOS) technology currently poses some difficult problems as the minimum feature size shrinks to below 100 nanometers and power supply voltage is reduced to less than 1.0V. A typical layout of a 0.18 micron transistor 100 is shown in FIG. 1. The transistor is manufactured over a well 110 where a diffusion area 120 is created. The gate 130 of the transistor 100 is formed over an island 120, and has a length “L” and width “w.” For example, 0.18 micron is the minimum gate length for a transistor in a 0.18 micron technology, with the width of the transistor varying for different circuit applications. Contacts 140 and 141 comprise one terminal of the NMOS transistor, for example the drain, and a contact 150 provides another terminal of the transistor 100, for example the source. The contact 131 is connected to the gate 130. There are other minimal feature sizes, such as a minimal size for the well “x,” and a minimum distance from the edge of the well to the diffusion area 120 marked as “y.” Dimensions, such as “w,” “x,” and “y” are generally process dependent. Power supply voltage is reduced in correspondence with the minimum feature size to maintain a limit on the electrical field across the oxide. Therefore, power supply voltage has decreased from 3.3V for 0.35-micron CMOS technology to 1.8V for 0.18 micron technology, and is further expected to be at the 1.0V level for 100 nanometers CMOS technology. While power supply voltage decreased, the threshold voltage of the NMOS transistors stayed between 0.45V and 0.35V. The relationship between the NMOS threshold voltage Vth and CMOS power supply VDD is known to be very critical. The threshold voltage determines the leakage current, Ioff, of the transistor when it is in its OFF state. As the threshold voltage is driven lower, the leakage current increases.
The drain current of the transistor is a direct function of the overdrive of the transistors, measured as the difference between power supply VDD and threshold voltage Vth. The drain current of the transistor determines the time required to charge the load capacitance from ground to the level of power supply VDD. This overdrive voltage has decreased constantly as the power supply decreased from 3.3V to 1.0V, while threshold voltage decreased only from 0.45V to 0.35V. For the 0.1 micron technology, the threshold voltage of the transistors is being scaled below 0.35V at the expense of very high OFF stage leakage current IOFF which ranges between 10 nA to 100 nA for a transistor with equal gate length and width, or W/L ratio of 1. For a transistor with a gate width to length (W/L) ratio of ten, the OFF current increases to ten times the value stated above, i.e. from 100 nA to 1000 nA. For a CMOS technology of 0.1-micron minimum feature size, a typical VLSI chip is expected to contain over 100 million gates. Given a leakage at every gate of 1 microamperes, there is 100 amperes of leakage current.
A scheme to control the threshold voltage dynamically has been proposed by Takamiya et al. in an article titled High Performance Electrically Induced Body Dynamic Threshold SOI MOSFET (EIB-DTMOS) with Large Body Effect and Low Threshold Voltage. Takimiya et al. suggest a scheme that shorts the gate and the substrate of the transistors, thereby causing the substrate voltage of the transistor to increase as the gate voltage is increased for a n-channel MOS (NMOS) transistor. This scheme is proposed for NMOS transistors fabricated on silicon-on-insulator (SOI) substrates, where the transistor substrate is totally isolated. This scheme manipulates the threshold voltage by changing the bias of the substrate in the positive direction for a NMOS transistor, along with a positive signal at the gate. As the substrate to source voltage becomes positive, the depletion layer width is reduced. This results in lower threshold voltage of the transistor, thereby increasing the current from the transistor. In the native form, the Takamiya et al. invention is applicable only for circuits using a power supply voltage of less than 0.6V because this scheme turns on the substrate-to-source diode, and the leakage from this diode must be limited. Otherwise one type of leakage would be traded for another, i.e. from drain-to-source leakage to substrate-to-source leakage. The changes in the threshold voltage of a MOS transistor, upon application of voltage to the substrate or the well region, is known as the body effect. A large body effect allows the changes in threshold voltage to be magnified upon application of bias to the substrate (well).
Douseki in U.S. Pat. No. 5,821,769 describes a method for the control of the threshold voltage of a MOS transistor by connecting a MOS transistor between the gate and the substrate to control the threshold voltage. The Douseki invention requires the addition of another transistor for every transistor whose threshold voltage is dynamically controlled. The adjusted threshold voltage is fixed by the power supply voltage and the threshold voltage of the additional transistor. The area penalty is fairly large for the Douseki invention and it requires additional process steps.
Notably, MOS technology has enabled the increase in the density of semiconductor memory chips with every step of scaling down of the minimum geometry of the transistors. The increase in density has taken place for dynamic random access memory (DRAM), static random access memory (SRAM), and non-volatile memory (NVM) chips. In fact, the memory chips have proven to be a major driver in shrinking geometry of MOS transistors. With the transistor minimum dimension scaled below 100 nanometers, the density of dynamic random access memory has reached four gigabits. The maximum current conducted by the transistor, ION is not increasing rapidly due to various second order effects that are becoming dominant. As a result, the transistors are being designed with lower threshold voltage VTH. This results in the increase of the leakage current IOFF.
FIG. 2 illustrates a typical SRAM element, also known as SRAM cell. This cell consists of two cross-coupled CMOS inverters consisting of NMOS transistors 222 and 224 and PMOS transistors 212 and 214. The memory cell is accessed using the pass transistors 226 and 228. These six transistors form a static random access memory. The PMOS transistors are formed in a n-well 232, which is shared by a certain number of SRAM cells. The cell is powered by the power bus, a metal line marked 242, and the ground connection is provided by the bus 244. The data to the cell are read and written by bit lines 246, representing the bit line value itself, and line 248 representing the inverse value of bit line 246, otherwise also know as a bit-line bar.
It is well known to those skilled in the art that SRAM cells, such as the SRAM cells shown in FIG. 2, are critically dependent upon the ratio of ION to IOFF, and a higher value of this ratio is desirable for the stability of the memory cell. With the lowered ratio of ION to IOFF, memory cells are not very stable and are subject to change of their state, or flipping, due to small electrical disturbances. Similarly, DRAM devices are also dependent upon the conducting characteristics of the MOS transistors. In a DRAM, a capacitor for storing the charge is used, and a transistor acts as a gating element to make or break the electrical connection to the capacitor. These pass transistors are designed with a high threshold voltage so that the off current of these transistors is low. This results in lowering of the ON current of these transistors, which leads to decrease in the read and write speed to the DRAM.
A similar problem also exists in other semiconductor memory types, including non-volatile memory, three-transistor SRAM, multi-port SRAM, and other types known to those skilled in the art. Prior art solutions have further attempted to address these issues. However, these approaches have caused significant increases in die area, reduced other beneficial characteristics of the devices, and require changes in the manufacturing process, as well as other limitations. There is therefore a need in the art for a technology which can reduce the leakage of memory cells using deep submicron MOS transistors, without adversely affecting other characteristics of the memory cell. Preferably such a solution will not change standard manufacturing processes and, preferably, such technology will be further applicable to multiple types of memory cells.